The present invention relates to a fuel assembly and more particularly to a fuel assembly suitable for application to a water cooling-type nuclear reactor, which can be attain a higher burnup, improve fuel economy and contribute to a desired thermal allowance.
A fuel assembly for a boiling water-type nuclear reactor generally comprises bundles of fuel rods each comprising a cladding and fuel pellets containing a fissile material, filled in the cladding, and a channel box having a square cross-section, which covers the fuel rods, as disclosed by a book "Light water reactor" written by Mamoru Akiyama and published by Dobun Shoin Publishing Co., Tokyo, Japan. Reactor core is charged with a plurality of the fuel assemblies. As fuel material, enriched uranium or mixed material of plutonium and uranium is used in an oxide form.
The reactivity of reactor core decreases with burning of the fissile material contained in the fuel material, and thus more fissile material than the critical mass is charged in the reactor core in the initial period of an operation cycle of a nuclear reactor so as to keep the nuclear reactor in a critical state even at the final stage of the operating cycle. The resulting excess reactivity is controlled by inserting a control rod of cross type cross-section containing boron carbide or hafnium between a plurality of the adjacent fuel assemblies and adding a burnable poison such as gadolinia, etc. to the fuel material, thereby adjusting the neutron absorption.
Recently, higher burnup is keenly desired from the viewpoint of prolongation of continuous operation period of a nuclear reactor and reduction in generation of spent fuel assemblies. In order to attain the higher burnup, it is necessary to increase the fuel enrichment, but the excess reactivity is inevitably increased thereby.
In the assemblies of the prior art, gadolinia (oxide of gadolinium) is used as a burnable poison. Gadolinium is characterized in that the excess reactivity can be controlled with a small amount of added gadolinium, because the thermal neutron absorption cross-section of odd nuclei (.sup.155 Gd and .sup.157 Gd) is considerably large, as shown in FIG. 3. However, gadolinium has a (n, .gamma.) nuclear reaction chain and a large resonance integrate of converted even nuclei (.sup.156 Gd and .sup.158 Gd) (see FIG. 3), and thus is a cause for neutron parasitic absorption. That is, when the excess reactivity increased by higher burnup is controlled only by gadolinia (strong neutron-absorbing substance), the reactivity of the reactor core is decreased by about 2% .DELTA.k/kk' due to the neutron parasitic absorption by gadolinia. Thus, it is necessary to charge the fissile material in excess correspondingly, and the fuel economy is deteriorated (first problem).
Furthermore, addition of gadolinia reduces the heat conductivity of pellets, and thus the enrichment of fissile material (.sup.235 U, etc.) in gadolinia-containing fuel rods is made lower than the maximum enrichment in a fuel assembly from the viewpoint of maintaining the soundness of fuel rods, thereby suppressing the power after burning-out of gadolinia. However, when the applicable uranium enrichment is limited, the average enrichment of a fuel assembly will be lower than the maximum uranium enrichment, and a higher burnup cannot be obtained (second problem).
FIG. 2 shows distribution of thermal neutron flux in the horizontal cross-sectional direction of a fuel assembly, where there is a difference in the thermal neutron flux at least by twice between the fuel rods at the respective corners of a channel box with softest neutron spectrum and the fuel rods at the second and third positions from the respective corners with hardest neutron spectrum. Such a structure gives excessively a large power from the fuel rods positioned in the peripheral region of a fuel assembly having a high thermal neutron flux, and thus the fuel rod power distribution in the fuel assembly is flattered by making the enrichment of fissile material (.sup.235 uranium, etc.) in the peripheral region than that of the center region. However, when the applicable uranium enrichment is limited even in that case, an average enrichment of a fuel assembly will be lower than the maximum uranium enrichment and no higher burnup can be obtained (third problem).
Technique of adding gadolinia to the fuel rods positioned in the peripheral region of a fuel assembly to flatten the fuel rod power distribution is disclosed in Japanese Patent Application Kokai (Laid-open) No. 62-32386. Even if the fuel rod power distribution could be flattened at the initial period of lifetime of the fuel assembly, the power of gadolinia-containing fuel rods will be increased with the progress of gadolinia burning, because gadolinia has a large thermal neutron absorption cross-section. Thus, it is difficult to flatten the fuel rod power distribution throughout the lifetime of a fuel assembly, and the above-mentioned problems 1 and 3 can never be solved thereby.
Technique of adding boron, a weak burnable absorber, to the fuel rods in the peripheral region of a fuel assembly to solve the above-mentioned problem 3 is disclosed in Japanese Patent Application Kokai (Laid-open) No. 57-196189. However, when the excess reactivity is controlled only with .sup.10 B having a thermal neutron absorption cross-section by about 1/100 smaller than that of .sup.157 Gd, a large number of fuel rods must be such burnable absorber-containing fuel rods. When all the fuel rods positioned in the peripheral region of a fuel assembly are such burnable absorber-containing fuel rods, the control rod worth will be decreased, if the fuel assembly is to be used in boiling water-type nuclear reactors.
Technique of adding boron to the peripheral region of a fuel pellet and gadolinia to the center region to solve the above-mentioned problems 1 and 2 is disclosed in Japanese Patent Application Kokai (Laid-open) No. 57-196189. In that case the number of burnable absorber-containing fuel rods can be reduced, but the problem 3 is not solved.